An axial piston pump of a type having a swash plate and a rotable cylinder block is known. An axial piston pump of the kind of a variable displacement which is carried out by changing an angle of the swash plate is shown in Japan Patent Application Laid-open No. 50681/1988.
According to the axial piston pump above, an integral rotation of the cylinder block and a rotary shaft makes a longitudinal stroke of a piston due to an existence of a swash plate and, resultantly, an end face of the cylinder block, which face being situated on the opposite side of a space in which a swash plate is placed, slides on a valve plate placed on a side of a cover. The valve plate has a pair of intake port and discharge port, respectively formed therein and they are placed oppositely along the diameter. When the piston moves or slides in the cylinder, liquid for example oil contained in the cylinder is intaken or discharged. The strokes of the piston are repeatedly carried out in the cylinder.
According to the conventional axial piston pump above, a pitch circle of the intake port and the discharge port (which circle is an imaginary one passing through these semicircular shape ports) is made large and accordingly such ports are placed on the circumference of the large pitch circle. It is apparent that the intake port and discharge port are formed in the cylinder block surface so as to extend in parallel with the piston.
Consequently, when the cylinder block rotates at a high speed, the intake port and the discharge port rotate at a high circumferential speed, so that an intake efficiency for liquid deteriorates and lowers and additionally disadvantageous vibration and noise are generated.
In order to solve the disadvantageous problem of the conventional axial piston pump, as shown in FIGS. 5 and 6, openings 80 of intaking and discharging, respectively connected to the cylinders are provided slantly so as to extend from ends of the cylinders toward the axis of the cylinder block 8. As a result, an intake port 700 and a discharge port 710 are placed near the central axis of the valve plate 7a.
However, according to the conventional construction of the pump, a diameter or an area of the pressure-receiving face of the valve plate 7a is small and a unit pressure applied to the valve plate 7a and the cylinder block 8. As a result, a sliding friction between the cylinder block 8 and the valve plate 7a increases, shortening the life time of the valve plate. In addition, a point of application of a pressing force F on the cylinder block 8 and another point of application of a reaction force F.sub.1 on the valve plate 7a are shifted, so a moment of force M shown in FIG. 5 is generated, resulting in a separation of the cylinder block 8 from the valve plate 7a, in particular, at a position C shown in FIG. 5 and in an increase of disadvantageous liquid leakage. It is possible to increase the diameter of the valve plate 7a to that of the cylinder block in order to solve the problem. Consequently, the circumferential speed of the large valve plate increases. Due to the composite effect of the moment of force M and the increased circumferential speed, it is apt to generate a burring or a seizure at a region or portions of an end of the valve plate and an outer edge of the cylinder block.